I'm fairly sure I read somewhere that JK Rowling herself considers The Order of the Phoenix to be too long.

Last time I re-read it (2010 I think) I thought so too.

I think I might have observed in another thread that, in contrast to modern series' like A Song of Ice and Fire, Professor Tolkien's work tends to leave the back-story to supplementary material, which is certainly where Sauron, for instance, gets much of his characterisation.

I don't think one approach is necessarily better than the other. Professor Tolkien's makes for a more focused narrative, I might argue.

I read an article recently that was compared science-fiction and fantasy to what the article-writer called "geekery", ie spec-fic writing that's more concerned about constant world-building, exploring how characters might behave in different situations within that world and generally self-indulgence of content at the expense of form and function, I suppose. It's sort of like the difference between asking "What does The Lord of the Rings argue about the human desire for permanence?" and asking "Would an Elf turn invisible if it wore one of the Nine or Seven?" "Who would win if Aragorn fought Eorl the Young?" etc.

Professor Tolkien didn't mind a bit of the latter himself I think, his letters and other writings are full of speculation on how things might have gone in Middle-earth as if it was a real, living place (as it more or less was to him) "What if Gandalf with the Ring fought Sauron without it?" "What would have happened if the War of the Ring was more like World War Two?" But he wisely kept those things speculative and didn't make them part of the plot. When they do appear, they serve to embellish the sense of realism without dominating the narrative.

I'm not saying that George RR Martin does these things, mind you, what I read of his work was certainly above that. I do believe, however, that it's one of the ways in which the logorrhea of authors can manifest.

The films are rife with this sort of thing incidentally, in deleted scenes and elsewhere. "What if Aragorn fought Sauron at the Black Gate?" "What if Thorin fought Azog?" "What if Radagast fought the Lord of the Nazgūl?"

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